Molding technique for fabrication of optoelectronic devices

ABSTRACT

Charge splitting networks for optoelectronic devices may be fabricated using a nanostructured porous film, e.g., of SiO 2 , as a template. The porous film may be fabricated using surfactant temptation techniques. Any of a variety of semiconducting materials including semiconducting metals and metal oxides (such as TiO 2 , CdSe, CdS, CdTe, or CuO) may be deposited into the pores of the porous template film. After deposition, the template film may be removed by controlled exposure to acid or base without disrupting the semiconducting material leaving behind a nanoscale network grid. Spaces in the network grid can then be filled with complementary semiconducting material, e.g., a semiconducting polymer or dye to create a exciton-splitting and charge transporting network with superior optoelectronic properties for an optoelectronic devices, particularly photovoltaic devices.

CROSS-REFERENCE TO AN EARLIER FILED APPLICATION

This application claims priority to U.S. Provisional Patent Application60/390,904, filed Jun. 22, 2002, the entire disclosures of which areincorporated herein by reference. This application is also acontinuation-in-part of U.S. patent application Ser. No. 10/290,119 toBrian Sager et al, entitled “OPTOELECTRONIC DEVICE AND FABRICATIONMETHODS”, filed Nov. 5, 2002, the entire disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to optoelectronic devices and moreparticularly to photovoltaic, e.g., solar cell, devices.

BACKGROUND OF THE INVENTION

Optoelectronic devices interact with radiation and electric current.Solar cells are a particular example of a useful class of optoelectronicdevices. Organic solar cell technology has been actively pursued in theresearch community due to its promise of lower cost, easiermanufacturability, and other potential advantages such as flexiblesheets of solar cells and various novel form factors.

Unlike Silicon solar cells, where photon absorption results in theformation of a free electron and hole, photoexcitation in organicsemiconductors leads to the formation of a bound electron/hole pair (an“exciton”). In most semiconducting (conjugated) polymers or smallmolecules, excitons are formed when exposed to radiation such as light.These excitons typically travel for about one exciton diffusion length(typically about 10 nm) before the electron and hole recombine returningtheir energy by emitting light and/or heat. The exciton diffusion lengthcan vary, depending on the specific organic compound, between 2 nm andseveral hundred nm.

To serve as a source for electrical energy, the electron and the holecomprising an exciton in an organic material must be separated so thatthe charges can be collected at different electrodes. To do so in anoptimal way, an exciton-splitting and transporting network must bestructured where the interfaces between electron- and hole-acceptingmaterials are spaced in, e.g., respective 10-nm arrays within the activearea of the solar cell device. At such interfaces, the electronstransfer into and move through the electron-accepting material, whilethe holes travel through the hole-accepting material.

Until recently, there have been only a few attempts to create a moreoptimal exciton-splitting and transporting network in an organic orplastic solar cell.

For example, Halls et al (Nature vol. 376, p 498, 1995) constructed aninterpenetrating mixture of organic polymers to increase the surfacearea between the electron and hole accepting materials. In particular,they mixed a blend of the conjugated polymers (i) soluble MEH-PPV (as ahole-acceptor) and (ii) CN-PPV (as an electron acceptor) in a ˜1:1 ratioto create an active layer in an organic photovoltaic device that showedan external quantum efficiency (EQE) of 6%. This EQE was two orders ofmagnitude higher than the single layer structures of MEH-PPV (0.04%) andCN-PPV (0.001%). Higher efficiencies were not obtained since the phaseseparating network was essentially random with isolated “islands”,phases/features that were too large (10-100 nm) and poor connectivitywith the respective electrodes.

More recently, Huynh et al. (Science, vol. 295, pp. 2425-2427, 2002)have reported the fabrication of hybrid nanorod-polymer solar cells.These cells have an EQE of 54%, a polychromatic efficiency of 1.7%, andare composed of a random assembly of CdSe nanorods inpoly-3(hexylthiophene). The totally random and highly inefficientplacement of the nanorods lowered the solar cell efficiency from whatwould be expected if the exciton-separating network was ordered andinterconnected on the desired 10-nm scale.

Granstrom et al. (Cavendish Laboratory) have shown that phase separationon a scale of about 50 nm can be obtained through lamination of twosemiconductive polymers giving polychromatic efficiency of 1.9% (Nature,vol. 385, pp. 257-260). The interpenetrating network obtained this wayis still not on the optimal size scale (about 10 nm) for these polymers.Conjugated polymers are known to be better hole conductors than electronconductors.

Similarly, in recent work at Cambridge University, Schmidt-Mende et al.(Science, vol. 293, pp. 1119-1122, 2002) made spatially organized thinfilms of perylene dye with a liquid crystal polymer, and achieved an EQEof 34%, a 1.9% polychromatic cell efficiency; however the efficiency waslow owing to the 100-200-nm scale of the interpenetrating dye/polymermixture used as a crude charge separating network.

In the solar cell devices constructed by these and other groups, thedevice architectures are sub-optimal compared to that needed forhigher-efficiency devices. These prior art devices are limited by theextent to which excitons can be harvested to perform useful work. Thisis due to two key factors:

First, in cells created with semiconducting nanorods, the nanorodswithin the solar cells were randomly arranged within a medium ofconducting polymer. Since many nanorods were only partially aligned andlarge clusters of nanorods (interspersed with areas of few rods) werepresent in the devices, many excitons traveling through the activelayers of these devices did not reach an electron affinity junctionbefore spontaneously recombining. As the spacing of the nanorods wasrandom, some areas of the device had many nanorods within 10 nm of oneanother, while many other areas of the device had no nanorods at allwithin 10 nm of one another (resulting in “dead” absorption space). Thisfactor decreased the efficiency of both electron and hole transfer atdifferential electron affinity junctions between nanorods and conductingpolymer.

Second, in cells composed of mixtures of perylene dye and liquid crystalpolymers, the rough 100-200 nm scale of the interpenetrating dye/polymerinterface resulted in low interfacial surface area, and thus the failureof many excitons traveling through such devices to reach an electronaffinity junction before spontaneously recombining.

Furthermore, the movement of electrons through the material requiredregularly and continuously spaced nanorods, which could collect andtransport free electrons to the outer boundary of the nanorod layer.This factor decreased the hole and electron collection efficiency. Allof these factors combine to reduce the efficiency of the device, andtherefore the potential electric power that can be produced by the solarcell.

An alternative approach to building an organic solar cell has beendeveloped by Michael Graetzel and his colleagues, who have constructeddye-sensitized, nanocrystalline TiO₂ based solar cells using a liquidelectrolyte (Kohle et al., Advanced Materials, vol. 9, p. 904, 1997). Inthis device structure, referred to herein as the “Graetzel cell”, 20 nmdiameter nanoparticles of TiO₂ are chemically bonded to Ruthenium dyemolecules. Upon absorbing light, the Ruthenium dye molecules inject anelectron into the titanium dioxide, which becomes positively charged asa result. Unfortunately, the Graetzel cell is relatively thick, e.g.,several microns in thickness. The TiO₂ nanoparticles are immersed in anelectrolyte. By immersing such a TiO₂ “paste” into a liquid redoxelectrolyte with I⁻/I₂ species, the positive charge of the dye moleculesis neutralized, closing the circuit. The Graetzel cell is known to beable to generally reach 10% polychromatic efficiency. The shortcoming ofthe Graetzel cell is its lack of long-term stability, with no solutionbeing known to effective seal the cell with the liquid I⁻/I₂ electrolytewhile remaining efficient and stable over several years. Furthermore,the three-dimensional charge splitting network in a Graetzel cell isessentially random, presenting many curves for the liquid electrolyte topenetrate. For example, a hole-accepting polymer can be incorporated asa replacement for the liquid electrolyte. Unfortunately, the polymercannot effectively be filled in uniformly in the particle region, asmany of the randomly formed spaces around the TiO₂ nanoparticles do notprovide effective access to the polymer to be incorporated into thefilm. Therefore, even if a Graetzel cell uses a solid electrolyte, thepore size distribution, pore spacing and pore filling are less thanoptimal.

Thus, there is a need in the art for optoelectronic devices, includingsolar cells that overcome the above disadvantages and a correspondingneed for methods and apparatus for producing such devices.

SUMMARY OF THE INVENTION

The disadvantages associated with the prior art are overcome byembodiments of the present invention directed to methods formanufacturing exciton-splitting and transporting networks foroptoelectronic devices.

According to an embodiment of the invention, an optoelectronic deviceincludes a nanoscale grid network and a network-filling material thatsubstantially fills spaces in the nanoscale grid network. Thenetwork-filling material and nanoscale grid network have complementarycharge-transfer properties with respect to each other. The nanoscalegrid network has interconnected structures of between about 1 nm andabout 100 nm in diameter that are distributed in a substantially uniformfashion with neighboring pores separated by a distance of between about1 nm and about 100 nm. The structures are interconnected and accessiblefrom an underlying layer and/or overlying layer (if any).

According to an embodiment of a method for making such an optoelectronicdevice, the nanoscale grid network may be formed on a substrate by firstforming a porous template, e.g., using surfactant temptation technique.After a pore-filling material is deposited in the pores of the poroustemplate, the template is removed, leaving behind a nanoscale gridnetwork. Spaces in the grid-network are filled with a network-fillingmaterial having complementary charge transfer properties with respect tothe material of the nanoscale network grid.

According to another embodiment of the invention, a solar powergeneration system may include an array of photovoltaic cells, whereinone or more cells in the array includes one or more porousexciton-splitting and charge transporting networks disposed between afirst electrode and a second electrode. Two or more cells in the arraymay be electrically connected in series.

Embodiments of the present invention provide new and usefuloptoelectronic devices including photovoltaic devices, as well as powergeneration systems that may be formed relatively inexpensively and on alarge scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts a solar cell according to the prior art

FIG. 1B depicts an exciton-splitting and charge transporting networkarchitecture that may be fabricated according to an embodiment of thepresent invention.

FIG. 2 is a cross-section schematic diagram of a device structure for anoptoelectronic device 200 according to an embodiment of the presentinvention.

FIG. 3 depicts a flow diagram illustrating an example of a method formaking a photovoltaic device of the type shown in FIG. 2 according to anembodiment of the present invention

FIGS. 4A-4H depict a series of schematic diagrams illustrating thefabrication of a photovoltaic cell according to the flow diagram of FIG.3.

FIG. 5 depicts a ternary phase diagram representing the synthesis of avariety of architectures within a porous film.

FIG. 6 depicts an isometric close-up view of a portion of a possibleporous film structure.

FIG. 7 depicts a graph of pore area vs. pore diameter for F127 templatedpolysiloxane porous film.

FIGS. 8A-8B depict atomic force micrographs of an example of asurfactant-templated porous film suitable for use with embodiments ofthe present invention.

FIGS. 9A-9B depict a solar power generation system according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Contents

I. Glossary

II. General Overview

III. Solar Cell Device Architecture

IV. Alternative Embodiments

V. Conclusion

I. GLOSSARY

The following terms are intended to have the following general meaningsas they are used herein:

Device: An assembly or sub-assembly having one or more layers ofmaterial.

Semiconductor: As used herein, semiconductor generally refers to amaterial characterized by an electronic bandgap typically between about0.5 eV and about 3.5 eV.

Hole-Acceptor, Electron-Acceptor: In the case of semiconductormaterials, hole-acceptor and electron-acceptor are relative terms fordescribing charge transfer between two materials. For two semiconductormaterials wherein a first material has a valence band edge or highestoccupied molecular orbital (HOMO) that is higher than the correspondingvalence band edge or HOMO for a second material, and wherein the firstmaterial has a conduction band edge or lowest unoccupied molecularorbital (LUMO) that is higher than the corresponding conduction bandedge or LUMO for the second material, the first material is ahole-acceptor with respect to the second material and the secondmaterial is an electron-acceptor with respect to the first material. Aparticular band edge or molecular orbital is said to be “higher” when itis closer the vacuum level.

Complementary charge-transfer properties: As used herein, a first andsecond semiconductor material are said to have complementarycharge-transfer properties with respect to each other when the firstmaterial is a hole-acceptor with respect to the second and the second isan electron-acceptor with respect to the first or vice versa.

Nano-Architected Porous Film: As used herein “nano-architected porousfilm” generally refers to a film of material having featurescharacterized by a width, or other characteristic dimension, on theorder of several nanometers (10⁻⁹ m) across. Nano-architected porousfilms may be produced by several techniques, including:

(a) Intercalation and/or grafting of organic or polymeric moleculeswithin a mineral lamellar network comprised of clays, phosphates,phosphonates, etc. The lamellar compounds serve as a network host whichpreserves the pre-established structural order. Organic molecules arethen inserted or grafted into this pre-structured network (which iscomprised of mineral(s)).

(b) Synthesis by electrocrystallisation of hybrid molecular assemblies.This synthesis technique drives the construction of highly organizedmineral networks with relatively long-range order that can be controlledand adjusted for electronic intermolecular transfer.

(c) Impregnation of preformed inorganic gels. In an example of thistechnique, a silica xerogel can be formed by hydrolysis andpolycondensation of silicon alkoxides with organic monomers (e.g. withmonomers that are susceptible to polymerization within the porous gelstructure). Methylmethacrylate (MMA) is an example of a suitable organicmonomer and the inorganic-organic hybrid obtained after polymerizationof the MMA has optical and mechanical properties often superior to theindividual components.

(d) Synthesis from heterofunctional metallic alkoxides metallic halidesor silsesquioxannes: Precursors of this kind have the formulaR_(x)M(OR′)_(n-x) or 3(R′O)Si—R″—Si(OR′)3, where R and R′ are eitherhydrogen (H), any organic functional group or a halide, R″ can be oxygenor an organic functional group, and M is a metal. Typically R and R′involve oxygen, e.g., —O—R and —O—R′. M may be any transition metal,e.g., titanium, zinc, zirconium, copper, lanthanum, niobium, strontium,or silicon, etc. The hydrolysis of alkoxy groups (OR′) followed by acondensation reaction will form the mineral network and the R groupswill imprint in the network the organic function.

(e) Synthesis of hybrid networks through the connection of well-definedfunctional nanobuilding Blocks. The pre-formatted species or buildingblocks could be in this case oxo-metallic clusters, nanoparticles (CdS,CdSe, . . . ), metallic or oxides colloids, organic molecules oroligomers. These blocks are functionalized during or after theirsynthesis with complementary species for tailoring the interface betweenorganic and inorganic domains. A review of this strategy has beenpresented in Comments in Inorganic Chemistry 20(4-6), 327-371 (1999),which is incorporated herein by reference.

(f) Templated growth of inorganic or hybrid networks by using organicmolecules and macromolecules as structure directing agents. In general,molecules like amines, alkyl ammonium ions, amphiphilic molecules orsurfactants can be used as templates to build a structured mineralnetwork. Materials of the zeolites families are among the mostintensively investigated systems. Molecular and supramolecularinteractions between template molecules (surfactants, amphiphilic blockcopolymers, organogelators, etc . . . ) and the growing hybrid ormetal-oxo based network permit the construction of complex hybridhierarchical architectures.

(g) Templated growth using nanoparticles, instead of surfactantsfollowed by removal of the nanoparticles, leaving behind a porousnetwork. The nanoparticles may be made, e.g., of latex, and removed,e.g., by heating the templated film to a sufficient temperature to “burnoff” the nanoparticles.

Surfactant Templation: In general, surfactant templation is a particularsubcategory of templated growth. As used herein, surfactant templationrefers an approach toward achieving pore size control of inorganic ororganic frameworks, e.g., by using surfactants or block copolymers astemplates to build a structured mineral network. Surfactant temptationmay be used to prepare a high-porosity surfactant-templated porous thinfilm. Surfactant temptation includes the sol-gel approach describedbelow.

Optoelectronic Device: A device that interacts with radiation andelectric current. Such a device could be a radiation-emitting device,e.g. a light-emitting diode (LED) or laser, or a radiation absorbingdevice, e.g. a photodetector/counter, photovoltaic cell (solar cell) orradiation-driven electrolysis cell.

Solar Cell: A photovoltaic device that interacts with radiation (oftenin the form of sunlight) impinging on the device to produce electricpower or voltage.

Organic Solar Cell: A type of solar cell wherein an active photoelectriclayer is fabricated, either partly or entirely, using organic materialscomprising, e.g., polymers, dyes, pigments (including mixtures) that arepredominantly carbon based compounds. These materials may be insulating,conductive or semiconductive.

Radiation: Energy which may be selectively applied includingelectromagnetic energy having a wavelength between 10⁻¹⁴ and 10⁴ metersincluding, for example, gamma radiation, x-ray radiation, ultravioletradiation, visible light, infrared radiation, microwave radiation andradio waves.

Material: The term “material” is used herein to refer to solid-statecompounds, extended solids, extended solutions, clusters of molecules oratoms, crystals, polymers, dyes, particularly including conjugatedpolymers and dyes.

Inorganic Materials: Materials which do not contain carbon as aprincipal element. The oxides and sulphides of carbon and the metalliccarbides are considered inorganic materials. Examples of inorganiccompounds include, but are not restricted to, the following:

(a) Intermetallics (or Intermediate Constituents): Intermetalliccompounds constitute a unique class of metallic materials that formlong-range ordered crystal structures below a critical temperature. Suchmaterials form when atoms of two metals combine in certain proportionsto form crystals with a different structure from that of either of thetwo metals (e.g., NiAl, CrBe₂, CuZn, etc.).

(b) Metal Alloys: A substance having metallic properties and which iscomposed of a mixture of two or more chemical elements of which at leastone is a metal.

(c) Magnetic Alloys: An alloy exhibiting ferromagnetism such as siliconiron, but also iron-nickel alloys, which may contain small amounts ofany of a number of other elements (e.g., copper, aluminum, chromium,molybdenum, vanadium, etc.), and iron-cobalt alloys.

(d) Inorganic polymers such as polysilanes or other non-carbon basedpolymers or monomers.

(e) Ceramics: Typically, a ceramic is a metal oxide, boride, carbide,nitride, or a mixture of such materials. Examples of such materialsinclude, among others, alumina, zirconia, Titania (TiO₂) siliconcarbide, aluminum nitride, silicon nitride

Organic Materials: Compounds, which principally consist of carbon andhydrogen, with or without oxygen, nitrogen or other elements, exceptthose in which carbon does not play a critical role (e.g., carbonatesalts). Examples of organic materials that can be synthesized using themethods of the present invention include, but are not restricted to, thefollowing:

(a) Organic Dyes and pigments such as perylenes, phthalocyanines,merocyanines, terylenes and squaraines and their derivatives.

(b) Polymers: Materials consisting of large macromolecules composed ofmore than one repeating units. Polymers, composed of 2-8 repeating unitsare often referred to as oligomers. Examples of such repeating unitsinclude, e.g., dyes or pigments. Polymers can be natural or synthetic,cross-linked or non-crosslinked, and they may be homopolymers,copolymers, or higher-ordered polymers (e.g., terpolymers, etc.).Examples of polymers include, but are not limited to, the following:polyurethanes, polyesters, polycarbonates, polyethyleneimines,polyacetates, polystyrenes, polyamides, polyanilines, polyacetylenes,polypyrroles, conjugated polymers, (e.g., semiconductive polymers suchas polyphenylvinylene, polythiophene, polyfluorenes, polyparaphenyleneand polymers containing C₆₀ or dyes such as perylenes orphthalocyanines) or conductive polymers such as doped PEDOT (Baytron),polyaniline or polyacetylene. These may be doped and may be synthesizedor grafted onto one another using either classical organic chemistrytechniques or using enzymes to catalyze specific reactions.

II. GENERAL OVERVIEW

Although the following detailed description contains many specificdetails for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the examples of embodiments of the invention described below are setforth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

As shown in FIG. 1A, a conventional exciton-splitting junction 100 is aflat plane, with a low interfacial surface area between ahole-transporting layer 101 and an electron-transporting layer 102. As aresult of the low interfacial surface area, the junction 100 has only amodest potential for harvesting excitons. To optimize the efficiency ofan exciton-splitting and charge transporting network, the interfacialsurface area between the hole- and electron-accepting materials shouldbe reachable for excitons. According to an embodiment of the presentinvention, this can be achieved as shown in FIG. 1B through the creationof a three-dimensional nanoscale exciton-splitting and chargetransporting network. 110. The exciton-splitting and charge transportingnetwork 110 has interpenetrating regions with complementarycharge-transfer properties, e.g., an electron-transporting material 111and hole-transporting material 112, that alternate in a substantiallyuniform fashion on a scale less than or equal to the exciton pathlength. The exciton-splitting and charge transporting network 110 isshown in simplified form for the sake of clarity. One possiblevariation, among others, of a configuration of an exciton-splitting andcharge transporting network 110 is shown in FIG. 6.

The exciton-splitting and charge transporting network 110 may be in theform of a porous nano-architected film having interconnected poresfilled with a pore-filling material, wherein the porous nano-architectedfilm and pore filling material have complementary charge-acceptingproperties. The porous nano-architected film may containregularly-spaced pores roughly 1 nm to 100 nm (more preferably 2 nm to50 nm) in diameter where neighboring pores are between about 1 nm andabout 100 nm (more preferably 2 nm to 50 nm) apart measured, e.g., fromnearest edge to nearest edge. The pores are preferably interconnectedwith each other and accessible from any underlying layer and/oroverlying layer. Such a porous nano-architected film may be a surfactanttemplated porous film. One approach to construct such asurfactant-templated porous film is to use sol-gel based self-assemblyof porous nanofilms to construct a mold having 1-100 nm diameter poresspaced 1-100 nm apart. The pores in the porous mold may be filled with asemiconducting material. The mold may then be etched away to leavebehind a semiconducting nanoscale grid network. The nanoscale gridnetwork may be filled with a semiconducting polymer or othersemiconductor material having complementary charge transfer propertieswith respect to the semiconducting material of the nanoscale gridnetwork.

III. DEVICE ARCHITECTURE

According to an embodiment of the present invention, an optoelectronicdevice includes (1) a low-cost, highly reproducible porousnano-architected film having interconnected pores filled with (2) apore-filling material having complementary charge-transfer propertieswith respect to the porous nano-architected film. The pore-fillingmaterial fills pores in the surfactant templated porous film to createan interpenetrating, nanoscale exciton-splitting and transportingnetwork for optoelectronic devices such as photovoltaic cells. Although,examples of this embodiment are described in terms of an electronaccepting porous film and a hole-accepting pore-filling material, thecharge transfer properties of the porous film and pore filling materialmay be reversed, i.e., the porous film may be a hole acceptor withrespect to the pore-filling material, which is an electron acceptor withrespect to the porous nano-architected film. Such an optoelectronicdevice may be a photovoltaic device, such as a solar cell.Alternatively, the optoelectronic device may be a radiation-emittingdevice, such as an LED, laser, etc.

FIG. 2 depicts an example of a device structure for an optoelectronicdevice 200 according to an embodiment of the present invention. Thephotovoltaic cell 200 generally includes a first electrode 202, acharge-transport film 204, an exciton-splitting and charge transportingnetwork 205 and a second electrode 210. The charge splitting networkincludes a nanoscale grid network 208 and a network-filling material206. The cell 200 may optionally be protected by encapsulants 212, 214.Furthermore, a first optional interface layer 203 may be disposedbetween the charge transport layer 204 and the first electrode toimprove charge injection into the first electrode. The interface layer203 may also improve mechanical properties such as flexibility andresistance to short circuits. A similar second optional interface layer209 may be disposed between the nanoscale grid network 208 and thesecond electrode 210.

The first electrode 202, which may serve as a base for the device 200,may be in the form of a commercially available sheet material such assuch as C-, Au-, Ag-, Al-, or Cu-coated Steel Foil. The charge transportfilm 204 and the network filling material 206 and may both be composedof the same material, e.g., an hole-accepting material such as aconjugated polymer or an oxide, such as copper oxide. Alternatively, thecharge-transport film 204 may be made from different materials. Thematerial used in the network-filling material 206 and/or chargetransport film 204 may be altered to optimize its radiation absorptionand/or charge transport properties, e.g., through carbon or hydrogendoping. The charge-transport film 204 provides electrical contactbetween the network-filling material 206 and the substrate electrode202.

The nanoscale grid network 208 contains substantially uniformlydistributed, e.g., regularly spaced, structures roughly 1 nm to 100 nmin diameter and more preferably, about 5 nm to about 15 nm in diameter.In general, neighboring structures are between about 1 nm and about 100nm apart measured, e.g., from nearest edge to nearest edge. Morepreferably, the pores are between about 5 nm apart and 15 nm apart, edgeto edge. Where organic semiconductors are used for either the nanoscalegrid network 208 or the network-filling material 206, it is desirablefor the size and spacing of the pores to be on the order of the excitondiffusion length in the respective organic semiconducting material. Thesubstantially uniform distribution of the pores enhances the overallconversion efficiency of the photovoltaic cell 200.

The exciton-splitting and charge transporting network 205 is shown insimplified form for the sake of clarity. Although the pores in theexciton-splitting and charge transporting network 205 may be alignedsubstantially parallel to each other as shown in FIG. 2, the pores mayalso be interconnected and, most importantly, accessible from bothneighboring layers, e.g. electrodes 202, 210 or interface layers 203,209. One possible variation, among others, of a possible configurationof the pores in an exciton-splitting and charge transporting network isshown in FIG. 6.

The network-filling material 206 fills the spaces between the structuresin the nanoscale grid network 208. The network-filling material 206 hascomplementary charge-accepting properties with respect to the nanoscalegrid network 208. The optional charge-transport film 204 inhibits orprevents direct contact between nanoscale grid network 208 and thesubstrate electrode 202. The nanoscale grid network 208 may cover thenetwork-filling material 206 in such a way as to inhibit direct contactbetween the network-filling material 206 and the second electrode 210.By way of example, and without loss of generality, where the nanoscalegrid network 208 may be made from an electron-accepting material, e.g.,Titania, (TiO₂) zinc oxide (ZnO₂), zirconium oxide, lanthanum oxide,niobium oxide, tungsten oxide, strontium oxide, calcium/titanium oxide,sodium titanate, potassium niobate, Cadmium Selenide (CdSe), CadmiumSulfide (CdS), or Cadmium Telluride (CdTe) as well as blends of two ormore such materials such as TiO₂/SiO₂ blends/hybrids.

The network-filling material 206 may be a semiconducting, e.g.,hole-accepting, organic material. In the case of a complementarystructure, the network-filling material 206 may be an electron-acceptingorganic material. Examples of suitable semiconducting organic materialsinclude conjugated polymers such as poly(phenylene) and derivativesthereof, poly(phenylene vinylene) and derivatives thereof (e.g.,poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV),poly(para-phenylene vinylene), (PPV)), poly(thiophene) and derivativesthereof (e.g., poly(3-octylthiophene-2,5,-diyl), regioregular,poly(3-octylthiophene-2,5,-diyl), regiorandom,Poly(3-hexylthiophene-2,5-diyl), regioregular,poly(3-hexylthiophene-2,5-diyl), regiorandom), poly(thienylenevinylene)and derivatives thereof, and poly(isothianaphthene) and derivativesthereof. Other suitable semiconducting polymers include organometallicpolymers, polymers containing perylene units, poly(squaraines) and theirderivatives. Other suitable semiconducting pore-filling materialsinclude organic pigments or dyes, azo-dyes having azo chromofores(—N═N—) linking aromatic groups, phthalocyanines including metal-freephthalocyanine; (HPc), perylenes, naphthalocyanines, squaraines,merocyanines and their respective derivatives, poly(silanes),poly(germinates),2,9-Di(pent-3-yl)-anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone,and2,9-Bis-(1-hexyl-hept-1-yl)-anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-1,3,8,10-tetrone.

Alternatively, the network-filling material 206 may be a hole-acceptinginorganic material, such as copper oxide or an electron-acceptinginorganic material in the case of a complementary structure.Furthermore, the network-filling material 206 may be a combination oftwo or more compounds, e.g., solubilized buckminsterfullerene (C₆₀)and/or a dye, such as perylene and/or a polythiophene derivative. Thecombination of the electron-accepting nanoscale grid network 208 and thehole-accepting network-filling material 206 creates a nanoscale,high-interfacial area exciton-splitting and charge transporting network205.

The second electrode 210 provides an electrical connection to thenanoscale grid network 208. Preferably, the substrate electrode 202, thesecond electrode 210 or both electrodes 202, 210 are made from amaterial that transmits radiation of interest to the photovoltaicprocess that takes place within the cell 200. Examples of suitabletransparent conducting materials for the electrodes 202, 210 includeconductive transparent oxides such as doped tin oxide (SnO₂) andindium-tin oxide (ITO).

The optional interface layers 203, 209 may be made from conductingpolymers, such as PEDOT or polyaniline. Alternatively, the interfacelayers 203, 209 may include a material, such as lithium fluoride (LiF)that improves charge injection into the electrodes 202, 210 or smoothesout the surface roughness of the electrodes 202, 210.

The optional encapsulants 212, 214 protect the cell 200 from thesurrounding environment. Examples of suitable encapsulant materialsinclude one or more layers of polymers, such as polyethyleneterephthalate (PET) and/or Mylar®. Mylar is a registered trademark of E.I. du Pont de Nemours and Company of Wilmington, Del. Inorganicmaterials, such as glass and metal foils may also be used for theencapsulants 212, 214. Ethylene vinyl acetate (EVA) may be used ineither case in conjunction with the other encapsulant. The encapsulants212, 214 may also include nitrides, oxides, oxynitrides or otherinorganic materials that protect against exposure to water or air. Theencapsulants 212, 214 may also absorb UV-light to protect organicmaterials disposed between the encapsulants 212, 214.

There are many possible variations on the basic photovoltaic celldescribed in the example above. For example, the nanoscale grid network208 may be made from metal oxides, other than TiO₂, or blends of metaloxides, or blends of TiO₂ and SiO₂ precursor materials or othersemiconducting compounds that are capable of accepting electrons fromthe network-filling material 206 and transporting the electrons.Furthermore, the nanoscale grid network 208 may be made from metaloxides or other semiconductor compounds (e.g., conjugated polymers ordyes) that are hole-acceptors with respect to the network-fillingmaterial 206. One example, among others, of such a semiconductormaterial is copper oxide (CuO). In such a case, the network-fillingmaterial 206 would be an electron accepting material with respect to thematerial of the nanoscale grid network 208.

III. FABRICATION OF PHOTOVOLTAIC DEVICES

Exciton-splitting and charge transporting networks of the typesdescribed above and optoelectronic devices of the type depicted in FIG.2 may be manufactured in accordance with an inventive method. An exampleof an embodiment of such a method is 300 illustrated by the flow diagramof FIG. 3 and the series of schematic diagrams shown in FIGS. 4A-4F.

The method 300 may begin at 301 by optionally depositing a firstelectrode 402, e.g. indium-tin-oxide (ITO) on a substrate 400, e.g.glass or PET (Mylar). At 302 a first interface layer 403 may optionallybe formed on the first electrode 402. At 303 an optional firstcharge-transport film 404 may be formed on either the first electrode402 or the first interface layer 403 as shown in FIG. 4A. Alternatively,a commercially available sheet material such as such as Au-, Ag-, Al-,C- or Cu-coated Steel Foil may be used as the first electrode 402 and/orsubstrate 400. The first charge transport film 404 may be ahole-accepting material such as any of those materials described abovewith respect to network-filling material 206. Subsequently, at 304, aporous nano-architected template film 406 having pores 407 is formed onthe first charge-transport layer 404 as shown in FIG. 4B. Such as a filmmay be formed, e.g., by surfactant temptation techniques. Theconfiguration of the pores 407 is shown in simplified form for the sakeof clarity. Although the pores 407 may be aligned substantially parallelto each other as shown in FIGS. 4B-4F, the pores 407 are preferablyinterconnected and accessible from both neighboring layers, e.g.electrodes or interface layers. One example, among others, of a possibleconfiguration of the pores 407 is shown in FIG. 6.

In particular, the template film 406 may be made from oxides, such assilica (SiO₂).Alternatively, other metal oxides, such as Titania (TiO₂)may be used. The template film 406 may be a made using a variety ofsurfactant temptation techniques such as evaporation inducedself-assembly, etc. Precursors for the template film 406 may be coatedon the first 402, e.g. by web-coating, dip-coating, spin-coating orspray coating based deposition. By appropriate choice of the coatingparameters, the porous template film 406 can be formed such that thepores 407 are roughly 2 nm to 100 nm in diameter and neighboring poresare between about 2 nm and about 100 nm apart. The porous template film406 is dried, annealed and, at 306, the pores 407 are then filled with asemiconducting pore-filling material 408, as shown in FIG. 4C. Thepore-filling material 408 may be made of the materials described abovethe nanoscale grid network 208 of FIG. 2. The pore-filling material maybe deposited in the pores by any suitable technique, includingelectrodeposition or chemical bath deposition. In a particularembodiment, the pore-filling material 408 may be TiO₂, Copper Oxide,ZnO₂, ZrO₂ lanthanum oxide, niobium oxide, tungsten oxide, strontiumoxide, calcium/titanium oxide, sodium titanate and potassium niobate,CdSe, CdS, or CdTe or blends of two or more such materialselectrodeposited in a porous SiO₂ template film.

At step 308 the porous template film 406 is removed, as shown in FIG.4D, e.g., by chemical etching in a solution of sodium hydroxide (NaOH),potassium hydroxide (KOH), or hydrofluoric acid (HF), without removingthe pore-filling material 408. In a preferred embodiment, the poroustemplate film 406 may be removed by etching in a solution of 5 M-10 MNaOH with suitable washing of any debris of SiO₂. The removal of theporous template film leaves behind nanoscale grid network 405 havingstructures 408A made from the pore filling material 408 that haveessentially the same size and shape as the pores 407. The structures408A in the nanoscale grid network 405 are separated by spaces 407A leftby the removal of the porous template film 406.

A 310 a network-filling material 406A fills the spaces 407A in thenanoscale grid network 405, as shown in FIG. 4E. The network fillingmaterial 406A has complementary charge transfer properties with respectto the structures 408A, i.e., with respect to the pore-filling material408. By way of example, if the pore filling material 408 and structures408A are made of an electron-accepting material such as Titania, CdSe,CdS, or CdTe, the network-filling material 406A may be a hole-acceptingorganic material, including conjugated polymers or dye such asthiophene, polythiophene, or phthalocyanine, poly phthalocyanine or ahole-accepting inorganic material, such as copper oxide. For acomplementary structure, i.e., where the pore-filling material 408 is ahole-acceptor, the network filling material 406A may be as suitableelectron-acceptor. The combination of the network-filling material 406Aand the nanoscale grid network 405 creates a nanoscale, high-interfacialarea exciton-splitting and transporting network layer.

At 312, a second charge-transport film 409, may optionally be applied tothe exposed surface of the nanoscale grid network 405 as shown in FIG.4F. In general, the second charge transport film 409 may be made of anyof the materials suitable for the pore-filling material 408, e.g., asdescribed above. At 314, the nanoscale grid network 405 is then put inelectrical contact with second electrode 410, (which may be formed at315) e.g. via the second charge-transport film 409. For example, thesubstrate 400, first electrode 402, nanoscale grid network 405,network-filling material 406A, etc., may comprise one section 401 of aphotovoltaic device shown in the lower portion of FIG. 4F.

A second section 415, shown in the upper portion of FIG. 4F, may bebuilt using a commercially-available sheet material 412, for example, ametal, such as steel or aluminum, which may be coated with a secondmaterial to enhance or optimize the work function of the secondelectrode 410. Alternatively, Mylar (Polyethylene Terephthalate, PET)may be used as a sheet material 412 for transparent substrate. The sheetmaterial 412 may be coated with a transparent conducting material (e.g.a layer of tin oxide or indium tin oxide) to form the second electrode410. An optional second interface layer 413 may be applied to the secondelectrode 410. An optional layer of charge-transport material 411 may beapplied to the transparent second electrode 410 or the second interfacelayer 413 to complete the second section 415. The optional layer ofcharge transport material 411 may be made from the same type of materialas the second charge transport film 409. The first and second sections401, 415 may then be laminated together at the homo-junction (dashedlines) between the second charge transport film 409 and the chargetransport material layer 411, as shown in FIG. 4G.

In an optional step 316 the resulting device may be encapsulated. Forexample, an encapsulant layer 414 may cover the exposed surface of thefirst electrode 402 as shown in FIG. 4F. Alternatively, the firstelectrode 402 may be mounted to the encapsulant layer 414 prior toforming non porous film 404 or the porous film template 406. Note thatin this example, the sheet material 412 also may be used as anencapsulant layer.

The key steps in the method 300 are forming the porous template film(304) and filling the pores in the porous film with pore-fillingmaterial (306), removing the porous template film to form the nanoscalegrid network (308) washing out any remaining debris, and filling thenanoscale grid network (310). Some approaches to accomplishing these twosteps along with approaches to other steps in the method 300 arediscussed in detail below.

A. Fabrication of Porous Template Film

With respect to step 304 if FIG. 3, there are several approaches toforming the porous nano-architected film described above. One suchapproach, among others, involves templated growth of inorganic or hybridnetworks, e.g., by surfactant temptation.

Examples of surfactant-templation techniques for producing porous filmsare described, e.g., by Brinker, et al in U.S. Pat. No. 6,270,846, thedisclosures of which are incorporated herein by reference. Oneparticular surfactant-templation approach, among others, utilizesevaporation-induced self-assembly (EISA) to form a meso-organizedliquid-crystal template. This process has been well developed for thefabrication of porous silica, where the substrate is first coated withsiloxane and surfactants in an ethanol solution. As the ethanolevaporates over a short time (typically 60-120 seconds), the moleculeswithin the siloxane-surfactant micelle rearrange themselves to minimizetheir collective energy level. This process continues as the moleculesfurther rearrange their nanoscale organization into highly regularliquid-crystalline mesophases. The resulting porous films contain a highand tunable density of regular, interconnected pores spaced in repeatingpatterns, with pores neighboring pores spaced approximately 5 nm apartand with pore diameters of about 5 nm, dependant on the choice ofsurfactant. This nanoscale architecture is highly reproducible, and canbe permanently fixed by heating. The resulting nanofilm is extremelystable and mechanically robust. Pore diameter and pore spacing may beadjusted by (1) choice of surfactant, (2) concentration of surfactant,(3) the use of block co-polymers, (4) temperature, (5) humidity level,(6) deposition procedure and speed, (7) concentration of siloxane, (8)use of a cosolvent, (9) use of swelling agents or some combination oftwo or more of (1), (2), (3), (4), (5), (6), (7), (8) and (9).

FIG. 5 depicts a ternary phase diagram representing the formation of avariety of architectures within a porous film. The structure circled inFIG. 5 depicts a simplified and somewhat idealized desirable morphologyfor the porous template film. A possible variation of this morphology isshown in FIG. 6, which depicts a portion of a surfactant-templatedporous film 600. The film 600 has numerous pores 601 that areinterconnected with each other. Furthermore, the pores 601 provide acontinuous path, through one or more pores, between a first surface 602and a second surface 603. The path through the pores 601 provides accessto the pores from a layer overlying or a layer underlying thesurfactant-templated porous film 600. When the pores are filled with asemiconducting pore-filling material, charges have a path to migratethrough the pore filling material from the overlying layer to theunderlying layer and/or vice versa.

In one embodiment, among others, the porous template film is fabricatedusing a precursor sol. To synthesize the sol, mixtures of one or morealkoxides, one or more surfactants one or more condensation inhibitors,water, and ethanol are combined. In one embodiment, among others, thesurfactant is a molecule wherein n is 20 and m is 70.

Examples of suitable alkoxides include polysiloxanes such astetraethylorthosilicate (TEOS). Examples of suitable surfactants includeHO(CH₂CH₂O)_(n)(CH₂CHCH₃O)_(m)(CH₂CH₂O)_(n)H, where the subscripts m andn are integers. A particular surfactant of this type is the blockcopolymerpoly(ethyleneoxide)-b-poly(propyleneoxide)-b-poly(ethyleneoxide)(EO20-PO70EO20), sometimes known commercially as Pluronic P123. ForPluronic P123, n=20, m=70, n=20 and the nominal molecular weight is 5750g/mol. Other suitable surfactants include hexadecyl trimethylammoniumbromide (CTAB), polyoxyalkylene ether (e.g. Pluronic F127), andpoly(oxyethylene) cetyl ether (e.g., Brij56 or Brij58) Pluronic is aregistered trademark of BASF Corporation of Ludwigshafen, Germany. Brijis a registered trademark of Atlas Chemicals of Wilmington Del.

For Pluronic F127, which is a triblock copolymer (PEO-PPO-PEO, having ann-m-n ratio of EO97PO69EO97, i.e., n=97, m=69, n=97. The nominalmolecular weight for Pluronic F127 is 12,600 g/mol. F127 is adifunctional block copolymer surfactant terminating in primary hydroxylgroups. It is a nonionic surfactant.

Brij 56 is polyoxyethylene 10 cetyl ether. Brij 58 has several synonyms,including poly(oxyethylene) cetyl ether, poly(oxyethylene) palmitylether, polyethylene oxide hexadecyl ether, and polyethylene glycol cetylether.

Examples of suitable condensation inhibitors include acids such ashydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃),etc., bases such as sodium hydroxide (NaOH), triethylamine, etc., andchelating agents, including acetyl acetone, alcohol amines, peroxides,etc.

Generally speaking, the molar ratios of the surfactant, condensationinhibitor, ethanol and water may be in the following ranges with respectto X, where X refers to the central element or inorganic network atom,e.g., Ti, Zr, Zn, Si, etc. in the alkoxide:

[Surfactant]/[X]: a molar ratio ranging from about 1×10⁻⁷ to about 0.1

[Ethanol]/[X]: a molar ratio ranging from about 3 to about 20

[Condensation Inhibitor]/[X]: a molar ranging ratio from about 1×10⁻⁵ toabout 5

[water]/[X]: a molar ratio ranging from about 1 to about 20.

By way of example, a precursor sol for a porous template of SiO₂ may beprepared from TEOS, a polar organic solvent, water and an acid, ahydrophobic compound such as polypropylene oxide (molecular weight ofapproximately 2000) and a surfactant, such as a polyoxyethylene ether orP123. The polar organic solvent can be any solvent that solubilizes theother reactants, particularly such solvents as alcohols, and moreparticularly, methanol, ethanol, propanol, butanol, tetrahydrofuran, andformamide or mixtures thereof. An initial silica sol may be prepared byrefluxing TEOS, ethanol, water and an acid, such as HCl, atapproximately 60° C. By way of example, the molar ratio of the TEOS,ethanol and acid may be about 1:3.8:1.5×10⁻⁵. The sol may be cooled toroom temperature and surfactant, CH₃(CH₂)₁₅(OCH₂CH₂)₁₀OH, in amountsranging from 0.6 g to 1.0 g, and the polymer, polypropylene oxide (PPO),in amounts ranging from approximately 0 g to 1.2 g, may be added to 5 mLof the sol, along with 0.8 mL of 1N HCl. The sols may be filtered and athin film prepared from this solution by spin-coating, web-coating,dip-coating, spray-coating, ink-jet printing, etc. onto a substrate.During the coating procedure, evaporation of the solvent causes theformation of surfactant-stabilized polypropylene microemulsionsincorporated into a surfactant-templated silica material. The as-coatedfilms must be crosslinked to form a mesoporous grid and may be heated toapproximately 400° C. to 450° C. for approximately 3 hours to removesurfactant and polypropylene oxide templates. Incubation temperature,ramp rate and total incubation time may be varied to optimize theproperties of the film.

After incubation of the sol mixture, a substrate, e.g., the firstelectrode 402 (e.g., ITO-coated glass or PET), is dipped in the mixtureand removed e.g., using an automated, custom-built dip coating apparatusor a commercially available web coating system. Upon removal from thesol, preferential ethanol evaporation concentrates the sol in water,non-volatile surfactant, and the TEOS component thereby forming a SiO₂surfactant-templated porous film. The progressive increase in surfactantconcentration drives the self-assembly of metal-alkoxide-surfactantmicelles and their further organization into liquid-crystallinemesophases.

The highly-ordered structure of the resulting liquid crystallites can bepermanently fixed through exposure to heat. After pattern deposition anddrying, the surfactant templates can be selectively removed by annealingthe surfactant templated porous film at a temperature (e.g., about 170°C. to about 400° C.) that is sufficient to covalently crosslink themesoporous matrix and/or is sufficient to decompose the surfactantmolecules while remaining within the thermal stability range of theunderlying ITO-coated glass or ITO-coated Mylar substrate. The annealingtime depends, partly, on the annealing temperature. In general, the highthe temperature, the shorter the time and vice versa. An annealingtemperature of about 250° C. or higher is preferable as this temperatureserves both to covalently cross-link the matrix and to pyrolyze thesurfactant out of the matrix within a relatively short time. Once thesol has been cross-linked to itself and the substrate, any remainingsurfactant may be removed by heating at more than about 350° C. or bysoaking the substrate in ethanol or another appropriate solvent.Alternatively, the film may be annealed for a shorter time at a highertemperature or for a longer time at a lower temperature. Furthermore,either as an alternative to annealing, or in conjunction with annealing,the surfactant template may be exposed to energetic radiation, such asultraviolet (UV) radiation, to facilitate crosslinking of the grid toform a mesoporous grid and to destroy the structure of the surfactantand make it easier to wash out.

The annealing preferably occurs before the deposition of anysemiconducting material, e.g. electron-accepting material such as TiO₂,CdSe, CdS, CdTe, etc., into the porous template film (step 306 discussedbelow), as it is this initial step that creates the porous structure.Thus any semiconducting material to be deposited in a later step willnot be affected by the annealing of the porous Titania film in thisprior step.

For photovoltaic applications, it is desirable to create uniform filmswith regularly spaced pores having diameters of between about 1 nm andabout 50 nm diameters, most preferably about 4 to 10 nm. The propertiesof the porous film 406, e.g., pore size and surface area, may beconfirmed by N₂ gas adsorption isotherms. Such isotherms may beobtained, e.g., at −196° C. using a Beckman Coulter SA 3100 SurfaceAnalyzer. Gas adsorption is considered an accurate method fordetermining surface area and pore size in the 2-200 nm range. In thismeasurement technique, inert gas molecules (N₂) are physisorbed onto thesurface at a constant temperature, and the amount of gas adsorbed as afunction of pressure is recorded as the adsorption isotherm. Monolayerformation on the sample by N₂ adsorbate occurs due to favorableadsorbate-adsorbent energies and enables BET (Brunauer, Emmett, Teller)surface area calculation. Multilayer formation occurs preferentially inthe pores as a result of capillary adsorbate-adsorbate condensation andenables pore volume/size determination prior to bulk adsorbate-adsorbateinteraction and condensation.

FIG. 7 shows data depicting the distribution of pore sizes in porousfilms created using the protocol described above. The graph in FIG. 7plots pore area vs. pore diameter for a F127 templated polysiloxaneporous film. Solutions containing F127 surfactant and polysiloxane weredropcast, dried, and annealed at 400° C. for two hours. A porous matrixcovering a lateral surface area of 78.5 cm² was scraped off and placedin a sample chamber for surface analysis. Data was acquired using aBeckman Coulter SA 3100 surface analyzer using liquid nitrogen; sampleswere analyzed using the BET and BJH algorithms. A narrow distribution ofpore sizes clustered around 6 nm shows that the pore structure isrelatively uniform within the film, and that this structure can besuccessfully measured using N₂ adsorption isotherms.

Alternatively, Atomic Force Microcopy (AFM) can be used to directlymeasure the sizes, orientations, and distributions of the pores in theporous film 306, and to monitor the two-dimensional orientation of bothinorganic and organic crystals, to characterize the surface roughnessdata of all films including the scratches and other defects. FIGS. 8A-8Bdepict atomic force micrographs of an example of a surfactant-templatedporous film manufactured using techniques of the type described above.In this particular example, a porous film of SiO₂ was formed on a F:SnO₂coated glass substrate using a sol mixture containing TEOS, PluronicP123 as a surfactant, HCl as a condensation inhibitor, water andethanol. The sol was filtered and a thin film was prepared from thissolution by dip-coating onto a glass substrate. Prior to dip-coating,the substrate was cleaned and then dried by rinsing with isopropylalcohol (EPA). FIGS. 8A-8B show phase contrast images, taken in tappingmode, of a nano-structured SiO₂ surface with features of about 10 nmdiameter. The images have been modified to reduce noise and enhancecontrast. Phase contrast is associated with changes in the viscoelasticproperties. For example, small phase, indicated by the darker areas ofthe images, typically indicates the presence of a soft material. Arelatively large change in phase, indicated by the lighter shaded areasin the images, may be associated with the presence of pores filled withair.

Although the forgoing embodiment utilizes a porous template film ofSiO₂, alternative embodiments may use other metal alkoxides to producemesoporous matrices made from materials other than SiO₂, e.g., TiO₂,ZrO₂, ZnO, etc. that are amenable to etching. Furthermore, blends of twoor more alkoxides with different central elements, e.g., Si, Ti, Zr, Zn,etc., may be used in precursor sols to produce porous template filmsmade from blends of two or more different oxide materials such as SiO₂,TiO₂, ZrO₂, ZnO, etc.

B. Filling Pores in Porous Template Film

As described above with respect to step 306, after construction of thesurfactant-templated porous SiO₂ film 406, the pores 407 within theporous film 406 are substantially filled with a semiconductingpore-filling material 408 such as Titania (TiO₂) Zirconia (ZrO₂), ZnO,CdSe, CdS, CdTe, CuO or any of the other materials described above withrespect to nanoscale grid network 208. For example, semiconductornanoscale grid networks comprised of CdSe, CdS, Titania (TiO₂) Zirconia(ZrO₂), ZnO, CuO and/or CdTe materials can be deposited into the pores407 in the porous template film 406, e.g., using standardelectrodeposition or chemical bath deposition techniques. In aparticular embodiment, the pore 407 can be filled by electrochemicallygrowing metal or semiconductor within the pore channels of the poroustemplate film 406. For example, a porous silica thin film may be coatedonto a transparent conductive substrate such as SnO₂, F:SnO₂, or ITO,whose surface serves as a working electrode. Electrodeposition drivesthe growth of a metal or semiconductor material within the pore channelsfrom the bottom conductive surface upward towards the top of the porechannels. The silica template may then be removed via standardwet-etching techniques, resulting in an ordered nanoscale grid networkwith an interconnective pore structure that replicates (in the inverseform) the original template of the silica framework structure.

As used herein, the term “substantially filled” generally means that thepore-filling material 408 fills a significant volume of a sufficientpercentage of the pores 407. It is often desirable to completely fill atleast some of the pores 407 with the pore-filling material 408. Ingeneral, the larger the percentage of completely filled pores the betterthe performance of the resulting photovoltaic cell device. Inparticular, porous thin films with pores ranging from 4-10 nm indiameter may be filled with an electron accepting, pore-filling material408 such as TiO₂, ZnO₂, ZrO₂, CdSe, CdS, CdTe.

One approach to filling the pores 407 of the porous film 406 is to useelectrodeposition where the ITO coating in the SiO₂ porous filmunderlayer serves as a conducting electrode to attract metal ions fromsolution and thus drive the formation of material within the pores 407.In the majority of areas where the pores are filled, the nanoscale gridnetwork 405 will have a very high interfacial surface area for highlyefficient exciton splitting, while for the incompletely filled pores,the device will nevertheless-still function as a single-layeroptoelectronic or photovoltaic device. In both circumstances, excitonscan be split, and the device can produce electric power. By maximizingthe number of pores that are filled, the exciton harvesting efficiencymay also be maximized.

Pore filling and pore composition may be tested with absorptionmeasurements and XPS depth profiling. XPS involves irradiating a solidin vacuum with mono-energetic soft X-rays and analyzing the energy ofelectrons emitted as a result. The electron energy spectrum may beobtained as a plot of the number of detected electrons per energyinterval versus their binding energy. Each element has a uniquespectrum, and so the spectrum from a mixture of elements isapproximately the sum of the peaks of the individual constituents. Sincethe mean free path of electrons in solids is very short, the electronsdetected by XPS mostly originate from only the top few atomic layers,making XPS a surface-sensitive technique. XPS may be done in conjunctionwith sputtering, e.g., magnetron sputtering, to remove material from thesurface of the device while doing the XPS analysis. In this way it ispossible to determine the chemical composition of materials as afunction of depth within the device.

C. Removal of Template to Form the Nanoscale Grid Network

As described above with respect to step 308, after the pores 407 in theporous template film 406 have been filled, the porous template isremoved, leaving behind the nanoscale grid network 405. Any suitabletechnique, such as chemical etching, may be used remove the poroustemplate film 406. The technique used depends, in part, on the materialof the porous template film 406 and the pore-filling material 408. Forexample wet chemical etching using either a strong acid (e.g., 1 M HF)or a strong base (e.g., 5 M KOH at room temperature or elevatedtemperature) as reacting agents may remove template films made of silica(SiO₂), without attacking network grid materials such as TiO₂, ZnO,ZrO₂, CdSe, CdS, CdTe or the underlying substrate material, e.g., glassor Mylar. Coating these substrate materials with etch-resistantconductive materials such as ITO provides the desired substrateelectrode 402. Generally, most metal foils are unsuitable as theunderlying substrate electrode 402 as the acid or base tends to attackthem. However, etch resistant metals or metals coated with anappropriate etch-resistant and conductive protective material may beused as the substrate electrode 402. The acid or base solution etchesaway the silica template film, leaving the nanoscale grid network andthe underlying substrate electrode 402 intact. Preferably, NaOH oranother alkaline solution is used as the etching agent. The alkalinesolutions may have a pH of about 9 or above. Alcoholic solutions of NaOHor KOH may also be used to improve the wetting properties of the etchingsolution.

D. Incorporating Complementary Semiconducting Material into the GridNetwork

As described above with respect to step 310, after the porous templatefilm has been removed, spaces 407A in the nanoscale grid network arefilled with a complementary network-filling material 406A. For example,semiconducting organic materials, such as conducting polymers may beapplied to the nanoscale network grid using spin or dip coating,creating an ordered structure composed of both semiconducting materialand conducting polymer. This material combination provides anexciton-splitting and charge transporting network that may form thebasis for a photovoltaic device, LED or other optoelectronic device. Theconducting polymer infiltrates through the spaces 407A between thestructures 408A in nanoscale network grid 405, forming an integrated,active layer that may serve as the basis for an organic-inorganic solarcell. Alternatively, any of the network-filling materials describedabove with respect to network-filling material 206 may be used to fillthe spaces in the network grid 405.

The inventors have determined that the performance of optoelectronicdevices improves dramatically where the network filling material is apolymer has been purified, e.g., using rotary evaporation. By way ofexample, and without limitation, regioregularpoly(3-hexylthiophene-2,5-diyl)(P3HT) may be prepared for use as thenetwork filling material 406A using the following procedure.

Using a Soxhlet extraction apparatus, P3HT (0.58 g) may be extractedwith anhydrous hexane, e.g., in a 500 mL round-bottom flask undernitrogen for 24 hours. The setup may be wrapped with aluminum foil toprotect polythiophene from light. The impurities in the polythiophenematerial can be removed through chemical extraction. After cooling down,the hexane solvent may be replaced with a chloroform solvent. P3HT canbe extracted with chloroform within about an hour. P3HT in chloroformmay be collected using a rotary evaporator under nitrogen protection andstored in a glove box. Optoelectronic devices made with P3HT preparedand purified as described above as the network filling material haveexhibited light absorption of about 90%.

As described in optional step 312, after the spaces 407A in the gridnetwork 405 have been filled with network-filling material 406A, thefilled grid network 405 may then be coated with the second chargetransport film 409, e.g. in the form of a layer of electron-acceptingmaterial of any of the types described above with respect to thenanoscale grid network 208. The second charge transport film 409 coatsthe charge splitting network grid 405 contacting electron-acceptingportions of the network grid. Any suitable coating technique may beused, e.g., dip coating, web coating, spin coating, spray coating, andthe like. The second charge-transport film 409 may be a sol-gel basednon-porous film deposited, e.g., using web-coating, dip-coating,spin-coating or spray-coating based deposition. The pore-fillingmaterial 408 and second charge transport film 409 may be made from thesame material or, alternatively, from different materials.

To make a workable optoelectronic device, e.g., a photovoltaic cell orLED, it is desirable to electrically contact the hole-accepting portionsof the network grid 405 to an electrode, e.g. via the second chargetransport film 409. As shown in FIGS. 4A-4H, it is possible to make suchcontact by building a photovoltaic cell device of the type shown in FIG.2 in two sections and then joining the two sections together. Up to thispoint in the construction process (step 312 in FIG. 3), the firstsection 401 a photovoltaic cell device has been built. In an alternativeexample of steps 314 and 316, which are described in detail below, thesecond section 415 may be built and the two sections are joined togetherto form a completed device of the type shown in FIG. 2.

E. Coating of Thin Aluminum Foil Electrode with Charge Transfer Material

By way of example the second electrode 410 may be made from a foilcomprised of C- or Cu-coated steel foil, which can also provide formechanical strength. The foil can be coated with a layer of chargetransport material 411 e.g., using a solution containing the same typeof semiconductor material used both to fill the spaces 407A and to coatthe filled exciton-splitting and transporting network layer 405. Again,any suitable coating process may be used, e.g., dip coating, webcoating, spin coating, spray coating and the like. Lamination may alsooccur within the interface layers 403, 413 or any other layer as long asa multilayer structure of the type depicted in FIG. 4G results.

F. Lamination of Device Sections

Lamination of a device formed in two sections may be done within orbetween any of the layers of the device. For example, as shown in FIG.4H, the first and second sections 401, 415 of a photovoltaic cell devicemay be attached to each other by lamination at the homo-junction(indicated by the dashed line) between the optional layer of chargetransport material 411 coating second electrode 410 and the secondcharge transport film 409 formed on the network grid 405 and networkfilling material 406A. Upon lamination, a complete photovoltaic devicestructure will have been constructed.

IV. ALTERNATIVE EMBODIMENTS

A. Solar Power Generation Systems

Other embodiments of the present invention may be directed to solarpower generation systems that utilize photovoltaic cells thatincorporate exciton-splitting and charge transporting networks of thetypes described above. An example of such a power generation system 900is depicted in FIGS. 9A-9B. The power system 900 generally comprises anarray 901 of photovoltaic cells 902 having features in common with thosedescribed above with respect to FIGS. 2 and 4A-4H. In particular, one ormore of the cells 902 in the array 901 includes one or moreexciton-splitting and charge transporting networks 905 disposed betweena first electrode 904 and a second electrode 906. Each charge splittingnetwork 905 includes a nanoscale network grid having uniformlydistributed nanometer-scale structures, with spaces between thestructures filled with a network-filling material. The network grid andthe network-filling material have complementary semiconductingproperties with respect to each other, i.e., one material acts anelectron-acceptor with respect to the other material, which acts as ahole-acceptor. The electron-acceptor would be in electrical contact withthe first electrode 904 and the hole-acceptor material would be inelectrical contact with the second electrode 906. An interaction betweenradiation and the exciton-splitting and charge transporting network 905generates an electrical voltage between the first electrode 904 and thesecond electrode 906.

To obtain higher aggregate voltages, two or more cells, e.g., cells 902,or groups of cells, may be electrically connected in series. Forexample, as shown in FIG. 9B, cells 902, 902A, and 902B are connected inseries, with the second electrode 906 of cell 902 connected to the firstelectrode 904A of cell 902A and the second electrode 906A of cell 902Aconnected to the first electrode 904B of cell 902B. There are manycommercial solutions to interconnect of solar cells, many of which canbe applied to the device structure. For example, standardscribing/etching techniques as used by many thin-film manufacturers canbe used. For instance, either the first electrode or the secondelectrode may be separated by laser grooving, mechanical separation(mechanical grooving), or a separating line (e.g., ˜0.5 mm wide) can beetched across a large area photovoltaic web using a macroscopic screen(no photolithography is required). Once the cells 902 are divided fromone another, they can be interconnected in series by overlapping thetransparent electrodes (which can be similarly divided) with the bottomlayer.

Organic solar cells often generate higher voltages than most inorganiccell structures, resulting in individual cell voltages between 0.7 and1.3 V, and thus these cells require fewer interconnects to obtain thehigher aggregate voltages as desired in many applications. Conventionalcells tend to generate only about 0.5 to 0.8V per cell; and, further,silicon-based cells are restricted to the common silicon wafer sizes sothat they need to connect many cells in parallel to obtain high currentsby covering a larger surface area.

The system may optionally include an energy storage device 908 connectedto the array 901. By way of example, the energy storage system may be inthe form of one or more batteries or one or more electrolytic cells forproducing hydrogen from water using electricity generated by the cells902 in the array 901. Alternatively, the cells 902 may be configured togenerate hydrogen from water directly using a radiation-drivenelectrolytic reaction. The storage device 908 may include a storage tankfor holding the hydrogen and/or a fuel cell for converting the energystored in the hydrogen back to electric power on demand.

The system 900 may be scaled as a stand-alone for a commercial orresidential facility. Alternatively, the system may include an optionalDC-AC converter 910 so that electric power produced by the system 900may be distributed over a conventional electric power grid. Because ofthe improved efficiency and lower manufacturing cost of photovoltaiccells of the type described herein the system 900 is potentially capableof producing electric power at a cost per kilowatt hour (kwh)competitive with conventional electric grid rates.

B. Alternative Approaches to Porous Nano-Architected Films

Although the above description describes formation of porousnano-architected films by a particular templated growth technique thatuses surfactants as a structuring agent, the present invention is notlimited to this technique alone. Porous nano-architected films forexciton-splitting and charge transporting networks or optoelectronicdevices may alternatively be fabricated by such approaches as: (a)Intercalation and/or grafting of organic or polymeric molecules within amineral lamellar network; (b) Synthesis by electrocrystallisation ofhybrid molecular assemblies; (c) Impregnation of preformed inorganicgels; (d) Synthesis from heterofunctional metallic alkoxides orsilsesquioxannes; (e) Synthesis of hybrid through the connection of welldefined functional nanobuilding blocks; (f) templated growth ofinorganic or hybrid networks by using organic molecules andmacromolecules other than surfactants, e.g., amines, alkyl ammoniumions, amphiphilic molecules, as structure directing agents, and (g)templating with nanoparticles, instead of surfactants, followed byremoval of the nanoparticles, leaving behind a porous network. Suitableadjustment of the result effective parameters in these techniques mayproduce a nano-architected film having interconnected pores that aredistributed in a substantially uniform fashion with neighboring poresseparated by a distance of between about 1 nm and about 100 nm, whereinthe pores have diameters of between about 1 nm and about 100 nm. Theinterconnected pores may be accessible from an underlying layer and/oroverlying layer (if any). The pores in a porous nano-architectedproduced by any of these techniques may be filled with a pore-fillingmaterial having complementary charge transfer properties as describedabove.

V. CONCLUSION

Embodiments of the present invention provide novel and usefuloptoelectronic devices, such as photovoltaic cell devices for use inelectric power production as well as methods for the manufacture of suchmaterials and power systems using such devices. The exciton-splittingand charge transporting networks and photovoltaic cells described hereinare potentially less expensive to manufacture than conventionalexciton-splitting and charge transporting networks, optoelectronicdevices and photovoltaic cells. It is to be understood that the abovedescription is intended to be illustrative and not restrictive. Manyembodiments and variations of the invention will become apparent tothose of skill in the art upon review of this disclosure. Merely by wayof example a wide variety of process times, reaction temperatures andother reaction conditions may be utilized, as well as a differentordering of certain processing steps. The scope of the invention should,therefore, be determined not with reference to the above description,but instead should be determined with reference to the appended claimsalong with the full scope of equivalents to which such claims areentitled.

1. A method for making an optoelectronic device, the method comprising:forming a porous film on a substrate by a templation technique, whereinthe porous film includes interconnected pores, wherein the pores aresubstantially uniformly distributed and wherein interconnections of thepores are found within the film, wherein the pores have diameters ofbetween about 1 nm and about 100 nm, and wherein neighboring pores areseparated by between about 1 nm and about 100 nm; substantially fillingthe pores in the porous film with a pore-filling material, removing theporous template film while leaving behind the pore-filling material thatsubstantially filled the interconnected pores of the film, therebyforming a nanoscale grid network comprising interconnected structures ofthe pore-filling material that are distributed in a substantiallyuniform fashion with neighboring structures separated by a distance ofbetween about 1 nm and about 100 nm, wherein the structures havediameters of between about 1 nm and about 100 nm; and incorporating anetwork-filling material into the nanoscale grid network, whereby thenetwork-filling material substantially fills the spaces between thestructures in the nanoscale grid network, wherein the nanoscale gridnetwork and the network-filling material have complementarycharge-transfer properties with respect to each other to provide acharge-splitting network in the optoelectronic device.
 2. The method ofclaim 1, wherein the templation technique is a surfactant templationtechnique and the porous film is a surfactant-templated film.
 3. Themethod of claim 2, wherein the surfactant-templation technique includes:disposing a sol on a substrate, wherein the sol includes one or morealkoxides, one or more surfactants, one or more condensation inhibitors,water, and ethanol, evaporating the ethanol from the sol to form thesurfactant-templated porous film, and covalently crosslinking the filmto form a mesoporous grid.
 4. The method of claim 3, wherein the sol isdisposed on the substrate by web coating, dip coating, spin coating orspray coating.
 5. The method of claim 3, further comprising: heating thesurfactant-templated porous film to a temperature sufficient todecompose the surfactant molecules while remaining within the thermalstability range of the substrate.
 6. The method of claim 3, furthercomprising: annealing the surfactant-templated porous film at atemperature below which the one or more surfactants is pyrolized, andwashing the one or more surfactants with a solvent.
 7. The method ofclaim 6, further comprising: annealing the surfactant-templated porousfilm before heating the surfactant-templated porous film to atemperature sufficient to decompose the surfactant molecules.
 8. Themethod of claim 3, further comprising exposing the surfactant-templatedfilm to energetic radiation to decompose the surfactant molecules. 9.The method of claim 3 wherein the one or more surfactants, one or morecondensation inhibitors, water, and ethanol are in molar ratios in thefollowing ranges with respect to a central element X in one or more ofthe alkoxides: surfactant/X: a molar ratio ranging from about 1×10⁻⁷ toabout 0.1, ethanol/X: a molar ratio ranging from about 3 to about 20,condensation inhibitor/X: a molar ranging ratio from about 1.0×10⁻⁵ toabout 5.0, water/X: a molar ratio ranging from about 1 to about
 20. 10.The method of claim 3, wherein the one or more alkoxides includetetraethylorthosilicate (TEOS), the surfactant ispoly(ethyleneoxide)-b-poly(propyleneoxide)-b-poly(ethyleneoxide), andHCl is the condensation inhibitor.
 11. The method of claim 3 wherein thealkoxide is titanium ethoxide, the surfactant ispoly(ethyleneoxide)-b-poly(propyleneoxide)-b-poly(ethyleneoxide), andHCl is the condensation inhibitor.
 12. The method of claim 11 whereinthe initial sol is prepared by refluxing the one or more alkoxides,ethanol, water and HCl, at approximately 60° C.
 13. The method of claim3 wherein the one or more alkoxides include a blend of two or morealkoxides having different central elements.
 14. The method of claim 2,wherein the surfactant-templation technique includes evaporation-inducedself-assembly.
 15. The method of claim 1, wherein the pore-fillingmaterial includes one or more materials selected from the groupconsisting of Titania (TiO₂), zinc oxide (ZnO₂), zirconium oxide,lanthanum oxide, niobium oxide, tungsten oxide, strontium oxide,calcium/titanium oxide, sodium titanate and potassium niobate, cadmiumselenide (CdSe), cadmium sulfide (CdS), and cadmium telluride (CdTe) andblends of two or more of these.
 16. The method of claim 1 wherein thenetwork-filling material includes one or more materials selected fromthe group consisting of poly(phenylene) and derivatives thereof,poly(phenylene vinylene) and derivatives thereof, poly(thiophene) andderivatives thereof, poly(thienylenevinylene) and derivatives thereofand poly(isothianaphthene) and derivatives thereof, organometallicpolymers, polymers containing perylene units, and poly(squaraines) andtheir derivatives.
 17. The method of claim 1 wherein the network-fillingmaterial includes one or more materials selected from the groupconsisting of organic pigments or dyes, azo-dyes having azo chromofores(—N═N—) linking aromatic groups, phthalocyanines, perylenes,naphthalocyanines, squaraines, merocyanines and their respectivederivatives, poly(silanes) and poly(germinates).
 18. The method of claim1, wherein the network filling material is a polymer material.
 19. Themethod of claim 18 further comprising purifying the network fillingmaterial before incorporating a network-filling material into thenanoscale grid network.
 20. The method of claim 19, wherein purifyingthe network filling material includes collecting the polymer material byrotary evaporation.
 21. The method of claim 1 wherein thenetwork-filling material includes an inorganic material.
 22. The methodof claim 21, wherein the inorganic material includes copper oxide oranother metal oxide.
 23. The method of claim 1 wherein substantiallyfilling the pores includes electrodepositing a material into the pores.24. The method of claim 1, further comprising: covering the porous filmwith a layer of charge-transport material, wherein the layer ofcharge-transport material contacts the pore-filling material thatsubstantially fills the pores in the porous film.
 25. The method ofclaim 1 further comprising: electrically contacting the charge-transportmaterial with an electrode.
 26. The method of claim 1 wherein thetemplation technique uses nanoparticles to form the porous templatefilm.
 27. The method of claim 1 wherein removing the porous templatefilm includes etching the porous template film with an acid or alkalinesolution without attacking the structure of an underlying substrate. 28.The method of claim 27 wherein acid or alkaline solution is a solutioncontaining sodium hydroxide (NaOH).
 29. A method for making anexciton-splitting and charge transporting network, the methodcomprising: forming a porous nano-architected template film on asubstrate, wherein the porous nano-architected film includesinterconnected pores that are substantially uniformly distributed andwherein interconnections of pores are found within the film, wherein thepores have diameters of between about 1 nm and about 100 nm, whereinneighboring pores are separated by between about 1 nm and about 100 nm,wherein the interconnected pores in the porous nano-architected film areaccessible from an underlying layer and/or overlying layer;substantially filling the pores in the porous nano-architected templatefilm with a pore-filling material; removing the porous template filmwhile leaving behind the pore-filling material that substantially filledthe interconnected pores of the film, thereby forming a nanoscale gridnetwork comprising interconnected structures of the pore-fillingmaterial that are distributed in a substantially uniform fashion withneighboring structures separated by a distance of between about 1 nm andabout 100 nm, wherein the structures have diameters of between about 1nm and about 100 nm; and incorporating a network-filling material intothe nanoscale grid network, whereby the network-filling materialsubstantially fills the spaces between the structures in the nanoscalegrid network, wherein the nanoscale grid network and the network-fillingmaterial have complementary charge-transfer properties with respect toeach other so that an exciton-splitting and charge transporting networkis obtained.
 30. The method of claim 29, wherein the porousnano-architected film is produced using one or more techniques selectedfrom the group consisting of: intercalation and/or grafting of organicor polymeric molecules within a mineral lamellar network; synthesis byelectrocrystallisation of hybrid molecular assemblies; impregnation ofpreformed inorganic gels, synthesis from heterofunctional metallicalkoxides or silsesquioxannes, synthesis through the connection of welldefined functional nanobuilding blocks, templated growth of inorganic orhybrid networks by using organic molecules and macromolecules includingsurfactants, amines, alkyl ammonium ions, or amphiphilic molecules, asstructure directing agents and templated growth with nanoparticlesfollowed by removal of the nanoparticles.